DONETSK, Ukraine — The ruins of the once gleaming and modern Donetsk airport, site of near relentless fighting in recent days, finally fell on Thursday to pro-Russian rebel forces who then paraded captured Ukrainian soldiers through the streets of the embattled city.

The airport, which had been claimed by both sides, is nonfunctional, the terminal and runways having been destroyed months ago. Nonetheless, it has retained high symbolic value in the continuing hostilities as the government’s last toehold in the city, the largest in the contested territory of southeastern Ukraine.

By dawn on Thursday, it was clear the Ukrainian Army was in a chaotic and bloody retreat, leaving behind their dead in the ruins of the main terminal, a Russian news video showed. Later in the day, in an official statement, Ukrainian military officials acknowledged that they had lost the battle.

In Donetsk, rebels forced a dozen captured Ukrainian soldiers to kneel on the streets near where artillery fire had gutted a trolley bus, killing at least eight people, encouraging passers-by to beat and spit on them.

“Fascists!” one old woman yelled at them. “Who are the terrorists now?”

The renewed violence, in Donetsk and at a remote checkpoint north of Luhansk, another major separatist-held city, threatened to plunge the region into ever-deepening chaos.

Deepening the sense that the region might be descending into a fresh period of bloodshed, Gen. Philip Breedlove, NATO’s top commander in Europe, said that the alliance’s analysts had noted the presence of sophisticated Russian military systems, electronic warfare and air defense systems, in the conflict zone. Previously, he said, the presence of these systems has been associated with an incursion of Russian troops and presaged a fresh round of fighting.

Speaking at a meeting of security officials in Kiev after the capture of the airport and public humiliation of its last, captured defenders, Ukraine’s president, Petro O. Poroshenko, vented frustration with a broken peace process.

“If the enemy does not want to abide by the cease-fire, if the enemy doesn’t want to stop the suffering of innocent people in Ukrainian villages and towns, we will give it to them in the teeth,” Mr. Poroshenko said in a statement on his website.

The Ukrainian Army and volunteer soldiers had held the airport through months of close combat. At times, Russian-backed rebels and Ukrainian soldiers occupied different floors in the same building. Holes in the floor and stairwells became front lines.

The turning point came when rebels exploded one of the floors, raining concrete and debris onto the heads of Ukrainian forces on the level below.

The airport fell as months of continual rebel shelling had destroyed all of its defensible positions, wrote Yuri Butusov, the editor of a Ukrainian military news portal, censor.net.

“The new and old terminals, the control tower and everything that could serve as a point of defense was destroyed,” he wrote. “And the airport is under direct fire, and the last surviving defenders left the new terminal only today.”

Image

A pro-Russian separatist fighter took cover from shelling on Thursday near the airport in Donetsk, eastern Ukraine.CreditManu Brabo/Associated Press

For Ukraine, the airport was laden with symbolic value as the site of the first in a string of military victories last summer that ended in August with a Russian intervention and cease-fire.

“Ukrainian soldiers defending the Donetsk airport were compelled to surrender what just a year ago was a wonderful, modern airport,” one volunteer unit, the Azov Battalion, posted on its website.

Andriy Lysenko, a spokesman for Ukraine’s military, said Ukrainian soldiers still defended some airport territory, without clarifying what. “The airport is still a battlefield,” he said.

In the city, an angry crowd of Donetsk residents pounced upon a Ukrainian soldier marched by rebel troops to the site of the bus-trolley explosion. People screamed angrily at the soldier as he was held by rebels, some reaching out to slap him across the face, or punch the back of his head. At one point, the crowd surged in so close that the soldier bent over to escape their pokes and punches and was pummeled on his back until the rebels holding him managed to squeeze him through the crowd and into the front seat of a waiting S.U.V.

Pro-Russian separatist leaders and senior Ukrainian government officials were quick to trade accusations over who was responsible for the assault on the bus, with the foreign ministries of Ukraine and Russia issuing nearly identical statements calling for an objective investigation.

A video taken in the aftermath of the early morning explosion showed the bus, which ran from an electrical connection to overhead lines, with its tires flattened, its sides punched in, filled with shards of concrete and stone and all of its windows either shattered or pocked with holes. A burned out car sat in the street nearby, and the building opposite had its front gate smashed and most of its windows blown out. There were conflicting reports of eight to 13 people killed.

Image

Pro-Russian separatists on the outskirts of Donetsk on Thursday.CreditAlexander Ermochenko/Reuters

In Moscow, the Russian foreign minister, Sergey V. Lavrov, issued a statement blaming the Ukrainian government and calling the bus attack “a crime against humanity, a rude provocation aimed at undermining the efforts on a peace settlement.”

Mr. Lavrov added: “It is becoming obvious that the party of war in Kiev, and its foreign patrons, are not stopped by fatalities. Everything must be done to stop the shelling by Kiev of towns in southeastern Ukraine and to prevent the further pointless casualties among the civilians.”

With both sides equipped with heavy weapons, artillery duels all too often miss by hundreds of yards or more, killing bystanders.

The strike in Donetsk came the day after President Poroshenko, addressing the World Economic Forum in Davos, Switzerland, held up a piece of wreckage from another public bus, destroyed in an artillery attack on Jan. 13 near the town of Volnovakha, in which 13 people were killed. In his speech, Mr. Poroshenko said that thousands of regular-duty Russian troops had been moved across the border into eastern Ukraine, escalating the conflict.

Fighting also intensified early Thursday in an area of remote Ukrainian checkpoints and small villages along the northern approach to the rebel stronghold of Luhansk. Ukraine military officials had said they were convinced they were facing regular Russian troops when the engagement began on Tuesday.

Late Wednesday night and into Thursday morning, both Checkpoints 29 and 31, north of Luhansk, and the nearby villages of Krymske and Nizhneye were subjected to persistent shelling. This was followed, Thursday afternoon, by what Ukrainian military officials described as a tank assault, which they said their forces were able to repel.

The three days of fighting, though, resulted in widespread damage to the two villages, said Yaroslav Galas, head of the department of communications for the regional administration. Only about 150 residents were left in Krymske, he said, about 10 percent of the usual population, and a huge fire at the local power station, visible for miles around, made it unlikely the village would ever be reinhabited, he said.

“You can say the situation there is an emergency,” Mr. Galas said.

A version of this article appears in print on , on Page A8 of the New York edition with the headline: Chaotic Retreat Follows Ukrainians’ Withdrawal From Donetsk Airport. Order Reprints | Today’s Paper | Subscribe